1. Field of the Invention
This invention relates generally to mobile communications and more particularly to the determination of the positions of mobile stations in a mobile communications network. This invention relates specifically to the calibration of base station time in order to maintain accuracy in the determination of the positions of the mobile stations.
2. Description of the Related Art
Mobile communications networks are in the process of offering increasingly sophisticated capabilities for locating the position of a mobile terminal of the network. The regulatory requirements of a jurisdiction may require a network operator to report the location of a mobile terminal when the mobile terminal places a call to an emergency service, such as a 911 call in the United States. In a Code Division Multiple Access (CDMA) digital cellular network, the position location capability can be provided by Advanced Forward Link Trilateration (AFLT), a technique that computes the location of the mobile station (MS) from the mobile station's measured time of arrival of radio signals from the base stations. A more advanced technique is hybrid position location, where the mobile station employs a Global Positioning System (GPS) receiver and the position is computed based on both AFLT and GPS measurements. A further application of the hybrid technique is when time obtained from a GPS-synchronous cellular network is used in obtaining the GPS measurements and computing the mobile station's position.
The accuracy of the location determined by either the AFLT or hybrid technique depends in part upon the precision of the time base in each base station transmitter. For example, the IS-95A (CDMA) standard, published by the Telecommunications Industry Association (TIA) of Arlington, Va., allows up to a 10 microsecond uncertainty in the time of transmission from the base stations to the mobile stations. IS-95A section 7.1.5.2 subtitled “Base Station Transmission Time” says: “All base stations should radiate the pilot PN sequence within ±3 μs of CDMA System Time and shall radiate the pilot PN sequence within ±10 μs of CDMA System Time. All CDMA Channels radiated by a base station shall be within ±1 μs of each other.” Because the wireless signal propagates at the speed of light, approximately 3×108 meters per second, a 10 microsecond offset in transmission time, translates to 3 kilometers in ranging error.
In order to maintain time synchronization between the base stations, the base stations can be synchronized to each other or synchronized to a common time base. For example, the Global Positioning System (GPS) is used as a common time base, and each base station may include a GPS receiver. The GPS system includes a constellation of 24 satellites (plus spares) in orbit 11,000 nautical miles above the earth. Each satellite has an atomic clock and transmits a carrier signal modulated by a pseudorandom code and a navigation message modulated at 50 bits per second. The navigation message transmitted by each satellite contains GPS system time, clock correction parameters, ionospheric delay model parameters, the satellite's ephemeris and health, and also almanac and health data for the other satellites. The GPS signals from four or more satellites can be used to compute the GPS system time and the GPS receiver's geographic location.
Although the GPS system can provide a stable time base for the CDMA system, the reference point for GPS system time is the GPS antenna at each base station, and the reference point for the CDMA system time is the CDMA antenna at each base station. Each base station can have a respective time offset between the GPS system time and the transmission of CDMA signals due to variations in propagation delay or phase shift from the GPS antenna to the GPS receiver, from the GPS receiver to the CDMA transmitter, and from the CDMA transmitter to the CDMA antenna. Therefore, to reduce ranging error in AFLT position determinations and to reduce timing and ranging error in hybrid position determinations, every base station has to be individually calibrated with special test equipment after the base station installation is complete. The result of this calibration process is a time offset for each base station pilot. The time offsets are stored in a data base accessible during the computation of position of the mobile stations. Any subsequent hardware change necessitates re-calibration of the base station and updating of the data base. All this represents a costly process.
There are other methods for synchronizing base stations to each other, based on combining the Pilot Signal Strength Message (PSMM) messages sent by mobile stations in soft handoff with round trip delay (RTD) measurements made by the base stations in the active set. With this method, the base stations can be made synchronous with each other; however, it is difficult to maintain overall synchronicity with GPS time across the network of base stations.
Currently, GPS receivers are being incorporated into mobile terminals in order to increase the accuracy of mobile terminal location determination. The GPS receivers can be autonomous and perform all GPS acquisition functions and position calculations, or they can be non-autonomous (also known as wireless assisted) and rely on the cellular network for providing GPS acquisition data and possibly performing the position calculations. By receiving GPS aiding data from the network, a mobile terminal with GPS capability can obtain the time and position data from the GPS satellites in about 10 seconds or less, during a typical phone call. Many, if not most, CDMA wireless phones having GPS capability are expected to be wireless assisted GPS receivers having hybrid capability of providing both GPS and AFLT position information upon the request of a serving base station handling a call from the wireless phone. The position location session can be MS-assisted or MS-based, depending on where the position computation takes place. In the MS-assisted case, the mobile station sends back raw or pre-processed measurement data to the base station. A network entity then computes the location. In the MS-based case, the position computation is performed in the mobile station.
Message protocols and formats for CDMA position location employing AFLT, GPS, and hybrid receivers, applicable to both the MS-based and MS-assisted cases, have been published in TIA/EIA standard IS-801-1 2001, Position Determination Service Standard for Dual-Mode Spread Spectrum Systems—Addendum, incorporated herein by reference. Page 4-43 of this standard specifies that each base station shall transmit a GPS reference time correction of the base station antenna transmitting the CDMA pilot pseudorandom (PN) sequence.
Another position location technique is where the measurements are made by a network entity, rather than by the mobile station. An example of these network-based methods is the RTD measurement carried out by the serving base stations. Measurements made by the mobile station may be combined with network-based measurements to enhance the availability and accuracy of the computed position.